Procedures for preparing photographic images in silver by diffusion transfer principles are well known in the art. For the formation of the positive silver images, a latent image contained in an exposed photosensitive silver halide emulsion is developed and almost concurrently therewith, a soluble silver complex is obtained by reaction of a silver halide solvent with the unexposed and undeveloped silver halide of said emulsion. Preferably, the photosensitive silver halide emulsion is developed with a processing composition in a viscous condition which is spread between the photosensitive element comprising the silver halide emulsion and a print-receiving element comprising, preferably, a suitable silver-precipitating layer. The processing composition effects development of the latent image in the emulsion and, substantially contemporaneously therewith, forms a soluble silver complex, for example, a thiosulfate or thiocyanate, with undeveloped silver halide. This soluble silver complex is, at least in part, transported in the direction of the print-receiving element and the silver thereof is largely precipitated in the silver-precipitating element to form a positive image thereon. Procedures of this description are disclosed, for example, in U.S. Pat. No. 2,543,181 issued to Edwin H. Land. See, also, Edwin H. Land, One Step Photography, Photographic Journal, Section A, pp. 7-15, January 1950.
Additive color reproduction may be produced by exposing a photosensitive silver halide emulsion through an additive color screen having filter media or screen elements each of an individual additive color, such as red or green or blue, and by viewing the reversed or positive silver image formed by transfer to a transparent print-receiving element through the same or a similar screen which is suitably registered with the reversed or positive image carried by the print-receiving layer.
As examples of suitable film structures for employment in additive color photography, mention may be made of U.S. Pat. Nos. 2,861,885; 2,726,154; 2,944,894; 3,536,488; 3,615,427; 3,615,428; 3,615,429; 3,615,426; and 3,894,871. Diffusion transfer film units are known wherein there is contained a positive transfer image and a negative silver image, the two images being in the same or separate layers on a common, transparent support and viewed as a single, positive image. Such positive images may be referred to for convenience as "integral positive-negative images", and more particularly as "integral positive-negative transparencies". Examples of film units which provide such integral positive-negative transparencies are set forth, for example, in the above-indicated U.S. Pat. Nos. 3,536,488; 3,894,871; 3,615,426; 3,615,427; 3,615,428; and 3,615,429.
In general, silver-precipitating nuclei comprise a specific class of adjuncts well known in the art as adapted to effect catalytic reduction of solubilized silver halide specifically including heavy metals and heavy metal compounds such as the metals of Groups IB, IIB, IVA, VIA and VIII and the reaction products of Groups IB, IIB, IVA and VIII metals with elements of Group VIA.
Particularly preferred precipitating agents are noble metals such as silver, gold, platinum, palladium, etc., and are generally provided in a matrix as colloidal particles.
U.S. Pat. No. 3,647,440, issued Mar. 7, 1972 discloses receiving layers comprising finely divided non-silver noble metal nuclei obtained by reducing a noble metal salt in the presence of a colloid or binder material with a reducing agent having a standard potential more negative than -0.30.
Copending application Ser. No. 649,201, filed Jan. 14, 1976 (commonly assigned), now abandoned and replaced by continuation application Ser. No. 69,282, filed Aug. 24, 1979 discloses and claims a receiving element for use in an additive color photographic diffusion transfer film unit which comprises a transparent support carrying an additive color screen and a layer comprising noble metal silver-precipitating nuclei and a polymer; wherein the nuclei are present at a level of about 0.1-0.3 mgs/ft.sup.2, and said polymer is present at a level of from about 0.5 to 5 times the coverage of said nuclei. Preferably, the noble metal is obtained by reduction of a noble metal salt or complex, and more preferably, the noble metal is palladium.
Copending application Ser. No. 897,942, filed Apr. 4, 1978 (commonly assigned), now U.S. Pat. No. 4,186,013, issued Jan. 29, 1980, discloses and claims a receiving element for use in a silver diffusion transfer film unit which comprises a support carrying a layer of noble metal silver-precipitating nuclei in a polymeric binder composition of polyvinyl alcohol and gelatin.
Copending application Ser. No. 897,943, filed Apr. 4, 1978 (commonly assigned), now U.S. Pat. No. 4,186,015, issued Jan. 29, 1980, discloses and claims a receiving element for use in a silver diffusion transfer film unit which comprises a support carrying a layer of noble metal silver-precipitating nuclei in a binder composition of hydroxyethyl cellulose and gelatin.
Enhanced image stability can be provided to silver images by the employment of noble metal compounds containing a noble metal below silver in the Electromotive Force Series of Elements. A preferred system also includes the employment of an .alpha.,.beta.-enediol silver halide developing agent. Film units and processes disclosing and claiming such stabilization systems are set forth in U.S. Pat. Nos. 3,704,126; 3,730,716 and 3,821,000.
In selecting the specific noble metal compound, consideration must be given to the ligand. For example, one must be selected that will not deleteriously affect the sensitometry of the film unit and one which will control the migration of the noble metal compound from its initial location in the film unit to the developed silver image at the appropriate rate and time. Thus, a noble metal compound migrating too slowly may result in poor silver image stability in that the delay would permit drying or separation of the silver image from the remainder of the film unit before sufficient transfer of noble metal compound to adequately stabilize the silver image. On the other hand, a compound moving too rapidly, i.e., before silver image formation is essentially complete, could interfere with the development of either or both of the positive and negative silver images thereby adversely affecting sensitometry.
This invention also provides novel noble metal compounds adapted to provide such enhanced stability to silver images which compounds are not taught or suggested by the prior art.